If the trees are a little in the distance you can try using one of the distortion effects mask to the trees. Heat distortion, fluid distortion, energy distortion, smoke distortion. This should be pretty quick and easy to try.

Anything visually close is likely going to need much more targeted and detailed animation. As suggested the new puppet warp effect in HF6 might give you what you are looking for.

Norman has good advice on using distortion effects. Try duplicating the layer, masking the foliage and trying heat or smoke distortion. Just a little, nice and slow, with diffusion turned down. Or, duplicate/mask and try Atomic Particles. Either method can get good foliage movements. Trunks are harder.

Explore other filters in the Warp group. Bezier Warp can add some motion. So can a bit of bulge.

A key thing here ia you'll probably need to mask your trees to a new layer to isolate them, and you might have to try to REMOVE the trees from the background layer. Depends on the original image.

yeah, I've been working on getting this to give a very subtle slow breeze motion. The tree will be tracked on a camera pan and won't show very long. The biggest issue is viewing the changes as this effect seems to slow everything Waaaayyy down.